The present invention relates to an electrical-characteristic evaluation apparatus for measuring electronic properties within a minute area.
In a known conventional method for measuring electronic properties within a minute area, the minute area is electrically connected to a macro electrode or a measurement probe by use of electric wiring.
An example of such a method is disclosed in, for example, Surface Science, 386 (1997), pp. 161-165. In the disclosed method, through vapor deposition employing a mask, a metal wiring line having a width on the order of microns is formed in such a manner that the wiring line extends toward a minute area within which electrical characteristics are to be measured.
Further, Nature, 393 (1998), pp. 49-52 reports a method in which characteristics of a carbon nanotube are measured by use of wiring connection. In this method, a carbon nanotube is evaporated onto a substrate having a previously formed wiring line to thereby be connected to the wiring line, and the electrical characteristics of the carbon nanotube are measured.
Meanwhile, there has been reported a method in which a metal probe having a sharpened tip is brought into direct contact with a minute area in order to measure electrical characteristics within the minute area. For example, a method using scanning tunneling microscopy is described in Oyo Buturi, vol. 67, No. 12 (1998), pp. 1361-1369.
Moreover, Japanese Patent Application Laid-Open (kokai) No. 10-56045 describes a method for measuring characteristics of an electronic element formed in a sub-micron area. In these methods, a probe is caused to approach a sample to a degree such that tunneling current flows between the probe and the sample, to thereby establish electrical connection between the probe and the sample in a minute area. Further, in order to reduce the contact resistance between the probe and the sample, after detection of tunneling current, feedback control of the probe position performed while using the tunneling current as a servo signal is stopped, and the distance between the probe and the sample is forcedly reduced before performance of measurement of electrical characteristics.
However, the above-described conventional methods involving formation of wiring lines cannot cope with a structure of nanometer size, because the width and pitch of wiring lines cannot be made less than 0.1 xcexcm, even when the latest semiconductor processing technique is used.
Further, since the electrical connection between a wiring line and a structure to be measured is established by means of simple adhesion, the contact resistance between the wiring line and the sample increases. For example, in the above-described measurement for a carbon nanotube, the contact resistance is estimated to be about 1 Mxcexa9. The resistance of a structure portion in which a quantized conductance appears is as high as several kxcexa9. Therefore, when resistance of such a structure portion is measured by the conventional method, there arises a problem in that the contact resistance is higher than a resistance to be measured.
Further, in the case in which a wiring line has been formed in advance, samples having different structures and sizes cannot be handled. In the case in which a wiring line is formed after placement of a sample, a wiring line that matches the sample can be formed; however, this method involves an extremely high possibility of the sample being damaged during formation of the wiring line, thereby preventing accurate measurement.
In the above-described method involving use of a sharpened probe, electrical characteristics are measured in a state in which the probe has been caused to approach a sample to a degree such that tunneling current flows between the probe and the sample. In such a case, the contact resistance is about 1 Mxcexa9 to 1 Gxcexa9. Therefore, even in measurement of a semiconductor sample, the very high contact resistance lowers the reliability and accuracy of the measurement. In view of this, in the conventional method, the probe is caused to further approach the sample by a predetermined distance, to thereby reduce the contact resistance.
However, in this case, since feedback control of the probe position is not performed, the positional relation; in particular the distance, between the probe and the sample may change during the course of measurement, due to temperature drift of the sample and other factors. In a region in which tunneling current flows between the probe and the sample (tunneling region), when the distance between the probe and the sample changes by 1 xc3x85, the contact resistance changes by one order of magnitude. Therefore, when feedback control of the probe position is not performed during the course of measurement of electrical characteristics, the positional relation between the probe and the sample is not guaranteed, with the result that the absolute value of a contact resistance contained in measurement results cannot be determined and is not guaranteed to be constant.
In view of the foregoing, an object of the present invention is to provide an electrical-characteristic evaluation apparatus which can bring a plurality of metal probes into contact with a minute area with low contact resistance.
In the present invention, position of the probe is controlled by means of atomic force microscopy in order to establish contact between the probe and a sample. Therefore, the probe position can be controlled during measurement of electrical characteristics. In order to reduce the contact resistance between the probe and the sample, the probe position is controlled to within a region such that the atomic force between the probe and the sample becomes repulsive.
Further, in the present invention, in order to enable a plurality of probes to approach a minute area, cantilevers are fabricated and used. Each cantilever has a metal probe which is provided at the free end of the cantilever and whose tip projects from the free end. This enables the plurality of metal probes to approach one another to a degree such that their tips do not come into contact with one another. That is, the probes can be caused to approach a minute area to a degree equivalent to that attained in the conventional method employing scanning tunneling microscopy. In the present invention, a technique for cutting/depositing a material by use of a focused ion beam is employed in order to form a metal probe at the free end of each cantilever.
By use of this technique, a metal probe whose tip has a radius of curvature of about several tens of nm and a length of about several tens of xcexcm is transplanted to the free end of the cantilever. In the present invention, in order to enable independent control of positions of the respective probes, variations in resistance of a resistor element which is formed on each of the cantilevers are used as means for detecting displacement of the cantilever.
In another method, a piezoelectric effect of a piezoelectric element formed on each cantilever is detected. In order to enable the detection, each of the cantilevers used in the present invention has, in addition to the resistor element or the piezoelectric element, two electrodes for detection of displacement of the cantilever, and one electrode for measurement of electrical characteristics of a sample.